A typical rotary joint consists of a fixed collimator holder and a rotatable collimator holder which are relatively rotatable to each other to allow uninterrupted transmission of optical signals through the rotational interface from collimators in any one of the holders to the collimators in the other holder.
A multi-channel fiber optic rotary joint typically utilizes a dove prism and a gear box as the de-rotating mechanism between the fixed collimator holder and the rotatable collimator holder. It typically rotates at half the rotational speed of the rotatable collimator holder; effectively de-rotation the image.
The dove prism relies on refraction to bend the optical beam as part of the de-rotation; which theoretically should not cause a problem. However, in practice this refraction is a source of chromatic dispersion because optical signals have a finite spectral range. Since the angle of refraction is a partial function of the wavelength itself, each discrete wavelength in the optical signal travels a different distance in the dove prism which causes part of the signal to effectively fall behind other parts of the signal. This signal lag ultimately determines the maximum frequency at which a signal can be sent through the fiber optic rotary joint. One way to envision this problem is to picture a square wave. Each peak of the wave represents a discrete piece of information and the width of each peak represents the amount of dispersion in the signal. Now the only way to send the same information faster is to reduce the distance between signal peaks; however, if the distance between peaks is reduced to the point that the waves start to overlap then the information in the two waves starts to become scrambled. Therefore, the frequency at which a signal can be sent is limited by the width of each wave, or the amount of dispersion that occurs in the signal.
Another effect of dispersion is it effectively limits the wavelengths at which a particular device can be used. One of the results of different wavelengths traveling slightly different distances through the prism is each wavelength will emerge at different angles on the other side of the prism. As such, a device tuned at 1625 nm will not perform the same if used at 1310 nm. The degree to which the performance will degrade is directly related to the difference between the wavelengths in question. As such the unit will have better performance the closer the operating wavelength is to the wavelength at which the unit was tuned. Conversely, the performance of the unit will degrade the farther the operating wavelength is from the wavelength at which the unit was tuned.
This phenomena also sets a floor for the lowest achievable insertion loss. The cause of this lower limit is that in practice every light source has a finite spectral range around the nominal wavelength. The slight variation in the distance traveled by each wavelengths in the spectral range cause slight variations in the focal point for each wavelength in the spectral range. This theoretically reduces the efficiency with which the opposing collimator could recapture the signal, thereby putting a theoretical limit on the lowest achievable insertion loss.
All of the aforementioned problems can be resolved by using a de-rotating prism that does not rely on refraction, such as the Pechan prism or the K-prism. However, as Ames correctly indicated in U.S. Pat. No. 5,157,745; the Pechan prism suffers from high back reflection. This problem leads Ames to conclude that the dove prism is preferable. The configuration embodied herein, solves the back reflection problem by applying an optical coating to the surfaces of the Pechan prism through which the optical signal must pass. This optical coating will eliminate/reduce the reflection at zero degree and function as a mirror at 45 degrees as well. Similarly, high reflection optical coatings can be applied to the surfaces of the K-prism through which the optical signal must pass for the same net result.